SnO2 (stannic oxide) has been used for quite some time in transparent electrodes and in gas sensors. For these purposes it is applied in the form of a thin layer onto a substrate. The SnO2 layer may be formed in a single step by thermolysis of a spray-deposited layer containing a suitable tin compound as the precursor for the SnO2. This type of layer, frequently applied to glass as the substrate, is in particularly suitable for transparent electrodes, as e.g. used in solar panels. Alternatively the SnO2 may be applied as preformed nanoparticles, e.g. as a dispersion, onto the substrate which is then again thermolysed to sinter together the preformed nanoparticles to a thin layer having a high specific surface derived from the surface of the nanoparticles. This type of layer, if applied to an electrode as the substrate, is particularly suitable for gas sensors.
SnO2 is quite insoluble in aqueous solvents of neutral to weakly acidic pH. Preformed SnO2 might thus be obtained by precipitation of an initially strongly acidic solution of a soluble stannic ion-containing salt by alkalinisation. The excess acid upon neutralisation however forms an excess of salt which might be incorporated into the precipitated SnO2. A more convenient way is then to start from stannous ion-containing salts. These have higher solubilities at only weakly acidic pH, but on the other hand they require an oxidation step from the stannous to the stannic oxidation state before the SnO2 can be precipitated. A very common and known such oxidant for stannous ions is atmospheric oxygen. Uncatalyzed oxidation of stannous ions by oxygen is relatively slow, its half life time is in the order of magnitude of some hours or days. In such a case the insoluble precipitated SnO2 forms crystalline structures. On the other side, if the oxidation of stannous ions is very fast (e.g. with a half-life time in the order of magnitude of microseconds or even nanoseconds), the spontaneous formed particles of SnO2 are no more able to form defined crystalline structures. The particles of SnO2 then tend to remain as nano-sized particles suspended or dispersed in the aqueous medium. Fast oxidation of stannous ions may be achieved either by a fast stoichiometric oxidant or by a catalytically accelerated oxidation by oxygen.
It was observed by Raschig in “Zeitschrift für anorganische and allgemeine Chemie” (Journal for inorganic and general chemistry), 155, pp. 225-240, 1926, that stannous ions derived from SnCl2 could be quickly oxidized by nitrite to a hydroxo-containing stannic species. These studies were however done under exclusion of air, thus under removal of athmosperic oxygen, since the aqueous SnCl2 solutions were freshly prepared from metallic tin and hydrochloric acid. They were also done at a molar ratio of stannous ion to nitrite of 1:1. Raschig did not observe the formation of a precipitate of SnO2 under his conditions. In view of the manufacture of his SnCl2 with hydrochloric acid this might be due to the still markedly acidic pH of his reaction solution, which prevented the precipitation (see also following paragraph).
Acta Chem. Scand. 16(1), pp. 221-228, 1962, discloses that under strongly acidic conditions of 3M HCl or 2M H2SO4 nitrite also oxidizes stannous chloride to stannic species. This publication again used oxygen-free media (the solutions were bubbled at all times with oxygen-free nitrogen, see middle of page 222). Also, under such acidic conditions no SnO2 could have precipitated, as is evidenced by a more recent publication, Geosystem Eng. 5(4), pp. 93-98, 2002, in its FIGS. 4, 6 and in particular 8.
U.S. Pat. No. 4,164,542 A describes a process for detinning tin-plated scrap in which the coat of metallic tin is dissolved in a strongly alkaline solution containing 18-30% sodium hydroxide and 2-10% sodium nitrite at temperatures of up to 236° F. to form sodium stannate. It is assumed that this process intermediately forms stannous ions from the tin metal, which are then further oxidized to the sodium stannate. The sodium stannate is then precipitated by cooling; the precipitate is addressed as “sodium stannate crystals”.
For the use of SnO2 as a semiconducting layer in the above mentioned applications it is advantageous to increase its conductivity, which has commonly been done by doping it with other elements such as indium, antimon, cobalt, manganese and in particular fluorine. Fluorine-doped SnO2 coated glass has been recognized as a cheap alternative to indium doped SnO2 because it is quite stable under atmospheric conditions, chemically inert, mechanically hard, high-temperature resistant and has a high tolerance to physical abrasion. A long-known process for preparing fluorine-doped SnO2 coated glass is by spraying an aqueous solution of stannic species also containing HF onto the glass which is pre-heated to several hundred degrees, which dries and calcines the sprayed layer to obtain the fluorine-doped SnO2 coated glass (see e.g. Key Engineering Materials 380, pp. 169-178, 2008). In later publications concerning fluorine doped SnO2 semiconductive layers the sprayable solution has customarily been made by mixing an alcoholic SnCl4 pentahydrate solution and an aqueous NH4F solution. The employed molar amount of fluoride (as HF or as NH4F) has been typically in the range of 0.5 up to 8 times the molar amount of tin (see e.g. Example 2, paragraph 64 of US 2008/0237760). Nanoparticulate fluorine doped SnO2 was prepared in a recent publication (J. Sol-Gel. Sci. Technol. 53, pp. 316-321, 2010) nanoparticulate fluorine doped by the sol-gel technique using SnCl2, HF and ammonia in a mixed aqueous/methanolic/acetylacetone solvent to obtain the gel, followed by filtration, washing free from chlorine ions until a test with AgNO3 solution did not form any AgCl precipitate, and calcination at 600-700° C. This publication did not use any explicitly added oxidants.
Generally, the prior art known to the inventors or the applicant studied the oxidation of stannous ions either by nitrite or by atmospheric oxygen. Insofar as it studied the oxidation by nitrite it is silent as to the reaction mechanism of the oxidation. If in the prior art a stannous ion-containing salt was used for preparing precipitated nanoparticulate SnO2, then according to the knowledge of the inventors and the applicant it was always the cheap and easily available SnCl2.
The instant invention aims to provide an improved process for the oxidation of stannous ions to stannic compounds, in particular for the preparation nanoparticulate SnO2.